Question: $ { {1} \times \left[\begin{array}{rr}{2} & {-1} \\ {3} & {1}\end{array}\right] = {?} } $
Solution: To multiply a matrix by a number, multiply each item of the matrix by the number. ${\left[\begin{array}{rr}{1}\times{2} & {1}\times{-1} \\ {1}\times{3} & {1}\times{1}\end{array}\right]}$ Now, simplify: ${\left[\begin{array}{rr}{2} & {-1} \\ {3} & {1}\end{array}\right]}$